Method for production of paper

ABSTRACT

The invention relates to a method for the production of paper or board, wherein retention aids are added to the stream of stock. Improved retention and more effective dewatering are achieved by adding to the stream of stock a cationic polymer solution and a suspension-form microparticle mixture composed of a swellable clay of the smectite group and a colloidal synthetic metal silicate in which the prevalent cation is magnesium.

[0001] The present invention relates to a method for the production ofpaper and board, wherein there is used as a retention aid in theretention system a solution of a cationic polymer together with amicroparticle mixture which contains a swellable clay of the smectitegroup.

[0002] At present, the use of microparticles in the retention system ofpaper production, in particular in the production of fine paper, is verycommon, the aim being to improve further the efficiency of theproduction process. The advantages of the adoption into use ofmicroparticles include improved retention, more efficient dewatering,and better formation. The most effective of the microparticles in useare colloidal silica-based microparticles of various types, solid orsol, and bentonite-like swellable natural materials belonging to thesmectite group of clays. Instead of, or in addition to, amicroparticulate compound it is possible to use as a retention aid inthe retention system polymers, which may be anionic, cationic ornon-ionic, and which are characterized by a high molecular weight. Theproblem involved with these compounds is typically excessiveflocculation, which deteriorates the optical properties of paper.

[0003] The silicates may be natural crystalline minerals or syntheticmaterials. Synthetic silicates have the advantage of better controllableproperties, in which case the efficiency of the microparticulatematerial used can be maximized. The colloidal synthetic silicates usedas retention aids in retention systems include, for example, colloidalsilica and polysilicate, aluminum silicates, and aluminum silicatesmodified with alkali metals and with alkaline-earth metals. The particlesize of these materials is typically a few nanometers or a few tens ofnanometers, and they are more expensive than, for example, bentonite.

[0004] The minerals of the smectite group of natural clays includemontmorillonite, beidellite, nontronite, saponite and sauconite, whichare composed mainly of aluminum silicates and some of which contain, inaddition to sodium, also other cations, such as magnesium, iron, calciumor zinc. Smectites also include hectorite and vermiculite, which are,instead, composed mainly of magnesium silicate and contain to a lesserextent also other cations. Natural clays are typically somewhat darkerthan synthetic materials, owing to impurities present in them.

[0005] Bentonite is a species of rock mainly composed of montmorillonite(Kirk-Othmer Encyclopedia of Chemical Technology, Part 6, 4^(th)edition, p. 394). However, the name bentonite is commonly also used ofcommercial products which contain mainly montmorillonite. Bentonite-typematerials have been used in paper production especially as materialsadsorbing impurities. Natural hectorite is mainly composed of magnesiumsilicate. In hectorite, some of the exchangeable sodium ions have beenreplaced by lithium ions. In addition the structure contains somefluoride.

[0006] Bentonite has been used as a retention aid in paper productiontogether with a cationic polymer in the patent U.S. Pat. No. 4,753,710of Allied Colloids. In the process according to the patent, a cationicpolymer, preferably polyethylene imine, a polyamine epichlorohydrinproduct, a polymer of diallyl dimethyl arrmonium chloride, or a polymerof acrylic monomers, was added to an aqueous cellulosic suspensionbefore the last shearing stage, and bentonite was added after thisshearing stage. Improved retention, dewatering, drying, and web formingproperties were thereby achieved. In the microparticle system accordingto the method there is used bentonite, which is available under thetrade name HYDROCOL.

[0007] Respectively, in the paper production method according to thepatent U.S. Pat. No. 5,178,730 of Delta Chemicals, there is added to thepulp before the shearing stage a cationic polymer, which is preferably atertiary or quaternary amine derivative of polyacrylamide, and after theshearing stage, before the headbox, there is added a natural hectoriteat a weight ratio of 0.5:1-10:1. It has been observed that thecombination of polymer and hectorite used in the method affects fillerretention and dewatering more effectively than does, for example,bentonite used in a corresponding manner. The method according to thepatent can be used in both alkaline and acid paper production recipes.

[0008] In the patent U.S. Pat. No. 5,876,563 of Allied Colloids, acationic starch together with a cationic polymer and an anionicmicroparticulate material is used as the retention aid. Themicroparticulate material suggested for use in this connection is, forexample, bentonite or colloidal silica or polysilicate microgels orpolysilicic acid microgels together with aluminum-modified colloidalsilica, or aluminum-modified polysilicate mnicrogel or aluminum-modifiedpolysilicic acid microgel, of which a suspension is formed.

[0009] In the application WO 99/14432 of Allied Colloids, themicroparticulate aid is preferably bentonite, colloidal silica,polysilicic acid, polysilicate microgel, or an aluminum-modified versionthereof.

[0010] In Finnish patents 67735 and 67736, there is used, together witha hydrophobic size, a retention aid combination which contains, togetherwith a polymer, preferably polyacrylamide, as an anionic component acolloidal silicic acid, bentonite, carboxymethyl cellulose orcarboxylated polyacrylamide.

[0011] The use of silicate microparticles together with a cationicpolymer in a retention system is described in the patent U.S. Pat. No.5,194,120 of Delta Chemicals. The prevalent cation in the syntheticamorphous metal silicate was Mg, and the polymer was preferably aternary or quaternary amine derivative of polyacrylamide, their weightratio being between 0.03:1 and 30:1. By the method, retention,dewatering and formation were improved by using smaller amounts ofretention aids than previously, and thus the costs were correspondinglylower.

[0012] According to our observations, when bentonite is used togetherwith polyacrylamide, it serves as an effective microparticulate materialin the retention system. Compared with this, a synthetic metal silicatein which the prevalent cation is Mg is, in a corresponding situation,not as effective as bentonite.

[0013] We have observed, surprisingly, that when there is used amicroparticle mixture in which the major part consists of bentonite orhectorite and to which a small amount of a synthetic metal silicatehaving magnesium as the prevalent cation is added, the said mixtureserves as a microparticulate material more effectively than does eithercomponent of the mixture, bentonite or hectorite or synthetic metalsilicate, separately.

[0014] According to the invention there is thus provided a method forproducing paper or board in such a manner that retention aids are addedto the stock stream passing to the paper machine headbox, the stockstream is directed to the wire, the stock is dewatered in order to forma paper web, and the paper web is dried, the method being characterizedin that the retention aids used are a solution of a water-solublecationic polymer and a microparticle mixture which contains, in the formof a suspension, a swellable clay of the smectite group and a colloidalsynthetic metal silicate, the prevalent cation in the synthetic metalsilicate in the suspension being magnesium.

[0015] The said swellable clay of the smectite group, hereinafter in thespecification referred to as clay material, is preferably bentonite orhectorite.

[0016] The microparticle mixture in the form of a suspension ispreferably prepared by mixing the said clay material, preferablybentonite or hectorite, and the said metal silicate together while dry.A suspension is made from the dry mixture by slurrying the dry mixturein water, preferably to a concentration of 1-20%, and especiallypreferably to a concentration of approx. 5%.

[0017] The microparticle mixture can be transported and stored in theform of a suspension, but preferably the microparticle mixture istransported and stored in a dry form, and a suspension is prepared fromit on site, immediately before use.

[0018] The proportion of the clay material in the microparticle mixturemay be 85-99% by weight and that of the metal silicate 1-15% by weight.Preferably the mixing ratio of the synthetic metal silicate to the claymaterial is 0.03-0.1. The total amount of the microparticle mixture tobe added to the stock is preferably at minimum 0.05%, especiallypreferably 0.1-0.25%, of the dry solids weight of the stock.

[0019] According to the invention, the retention aids are preferablyadded in steps so that first the solution of a cationic polymer isadded, whereafter there follows a shearing process step for breaking upflocs, and thereafter the microparticle mixture in suspension form isadded.

[0020] By the use of the microparticle mixture according to theinvention, a surprisingly good retention is achieved, although when theclay material or the synthetic metal silicate is used alone as aretention aid, the retention result remains poorer. It can be assumedthat the synergy advantage is based on the ability of the simultaneouslyadded silicate to promote a more uniform distribution of the claymaterial particles into the aqueous phase, whereupon the surface area ofthe clay material particles can be exploited more effectively. When themicroparticle mixture according to the invention is used as a retentionaid, the filler retention may be up to 5 percentage points better thanwhen the individual components of the mixture are used in the sameamounts of dosage. A similar result is obtained for the total retention,even though the change is not as clearly observable as regarding thefiller retention, since filler constitutes most of the stock fractionmore difficult to retain on the wire.

[0021] The reproducibility of the measuring results is especiallysignificant; without exception, a better retention result is alwaysobtained with the mixture, regardless of the nrodiiction conditions,than with the individual components of the mixture

[0022] Furthermore, the color of the microparticle mixture is somewhatlighter than that of pure bentonite.

[0023] By the use of the microparticle mixture according to theinvention a high retention is attained by using a smaller amount ofretention aid as compared to the use of the individual components of themixture. In this case, for example, dust problems and the consequenthandling problems are smaller. The efficiency ratio of the use ofmicroparticles is improved as the attained efficiency can be maintainedconstant and the amount of material to be added can be reduced.

[0024] The synthetic metal silicate according to the invention must havea sufficiently high and preferably controllable cation exchangecapacity. Typically the exchangeable cation may be, for example, Li⁺.The prevalent cation is magnesium, as in, for example, the product soldunder the trade name of Laponite. The clay material may be anycommercial bentonite or bentonite-type material, such asmontmorillonite, beidellite, nontronite, saponite, sauconite,vermiculite or hectorite, or a chemically modified version of these.Advantageously bentonite can be used, for example, the Kemira Chemicalsproduct sold under the trade name of Altonit SF or natural hectorite.

[0025] The cationic polymer used in the invention can be producedadvantageously by copolymerizing acrylamide with a cationic monomer ormethacrylamide with a cationic monomer. The molecular weight of thecationic polymer is preferably at least 500,000, and it is added to thestock preferably in an amount of at minimum 0.02%, especially preferably0.03-0.05%, of the dry solids weight of the stock.

[0026] The cationic polymer used in the invention may be any copolymerof acrylamide and/or methacrylamide, prepared using at least as one ofthe comonomers a cationiccally charged or cationically chargeablemonomer. Such monomers include methacryloyloxyethyltrimethyl ammoniumchloride, acryloyloxyethyltrimethyl ammonium chloride,3-(methacrylamido)propyltrimethyl ammonium chloride,3-(acryloylamido)propyltrimethyl ammonium chloride, diallyldimethylammonium chloride, dimethylaminoethyl acrylate, dimethylaminoethylmethacrylate, dimethylaminopropylacrylamide,dimethylaminopropylmethacrylamide, or a similar monomer. The polymer mayalso contain monomers other than acrylamide, methacrylamide, or somecationic or cationizable monomer.

[0027] The cationic polymer may also be a polymer which has been treatedafterwards to render it cationic, for example, a polymer prepared frompolyacrylamide or polymethacrylamide by using Hofmann or Mannichreactions.

[0028] The cationic polymer may be prepared by conventionalradical-initiation polymerization methods, and as a product it may beeither dry powder or an emulsion of a polymer solution in an organicmedium.

[0029] Before dosing, preferably an 0.05-0.5% solution, especiallypreferably an 0.1-0.3% solution, is prepared of the polymer, whichsolution may be further diluted before the feeding point in order toensure good mixing.

[0030] The method according to the invention was observed to be robustwith respect to various test arrangements, pulps, and fillers. The stockmaterial and its initial pulp may, for example, be composed of aconventional chemical pulp or mechanical pulp or of other conventionalraw materials used in paper making, such as recycled paper. The filler,which may be, for example, ground or precipitated calcium carbonate,kaolin, calcined kaolin, talc, titanium dioxide, gypsum, syntheticinorganic or organic filler, preferably, however, calcium carbonate, isincorporated into the pulp by a conventional method before the adding ofthe cationic polymer. The method according to the invention can be usedin any conventional paper- or board-making apparatus. Furthermore, themethod is not critical as regards the effect of the synthetic metalsilicate type or of the mixing ratio of bentonite and metal silicate.

[0031] By the method according to the invention, retention can beimproved further as compared with prior known methods and, at the sametime, if so desired, the amount of the required retention aid can bereduced, whereupon any detrimental effects caused by its use areslighter.

[0032] The invention and its embodiments are described below with thehelp of various examples; the purpose of the examples is, however, notto restrict the scope of the invention.

EXAMPLE 1

[0033] Retention tests were carried out using a Dynamic Drainage Jar(DDJ) apparatus. The stock used was stock taken from a fine-papermachine, passing to the headbox. The stock sample had been taken justbefore retention aid additions. The filler content of the stock was 36%of the dry solids content of the stock. The filler was precipitatedcalcium carbonate. For the tests the stock was diluted withion-exchanged water from the original consistency of 8.7 g/l to aconsistency of 8.0 g/l. The pH of the stock was 8.1. The following,stepwise procedure was used in the tests:

[0034] 1. At time 0 s, the mixing velocity being 1500 rpm, the stocksample was poured into a vessel.

[0035] 2. At 10 s, the polymer was dosed into the stock.

[0036] 3. At 30 s, the mixing velocity was lowered to 1000 rpm.

[0037] 4. At 35 s, the microparticulate material or the microparticlemixture was dosed into the stock.

[0038] 5. At 45 s, a filtrate sample was taken.

[0039] The wire used was a 200-mesh DDJ wire 125P. The polymer was aKemira Chemicals cationic polyacrylamide (PAM1), which is a copolymer ofacrylamide and acryloyloxyethyltrimethyl ammonium chloride and has acharge of approx. 1 meq/g and a molecular weight of approx. 7 Mg/mol.The bentonite used was Altonit SF of Kemira Chemicals. The syntheticmetal silicate used, in which the prevalent cation was magnesium, wasLaponite RD of Laporte. The dosages are indicated as the amount of thematerial dosed per dry solids weight of the stock, the unit beingg/tonne. In the microparticle mixtures the mixing ratio is indicated inpercentages by weight. The mixture contained bentonite 95% and syntheticmetal silicate 5%. The retention results are shown in Table 1. TABLE 1Total retention and filler retention results both when bentonite andwhen a mixture of bentonite and a synthetic metal silicate (mixture) wasused PAM1 Microparticle Filler retention, % Total retention, % g/tonneg/tonne Bentonite Mixture Bentonite Mixture 250 1000 16.4 18.9 64.8 67.6250 2000 19.2 20.8 64.8 69.5 400 1000 31.0 31.5 71.2 71.6 400 2000 38.342.7 74.3 77.5 500 1000 38.9 47.7 75.1 79.4

[0040] With all PAM1 dosages it can be observed that the mixture of asynthetic microparticulate material and bentonite works with the samedosages better than does bentonite alone. At its most advantageous themixture is, in the stock used here, at the highest PAM1 dosages (500g/tonne), in which case a clear improvement is seen especially in thefiller retention.

[0041] This example shows clearly that the retention results are alwaysreproducibly better when a mixture of bentonite and a synthetic metalsilicate is used than when bentonite alone is used.

EXAMPLE 2

[0042] Retention tests were performed mainly in the same manner as inExample 1, but the fine-paper machine used was not the same, and so thenumerical values are not directly comparable with the values given inExample 1. The stock used was an artificial stock prepared in thelaboratory, for which bleached chemical pine and birch pulps, used at aratio of 1:1, were taken as a thick pulp from a fine-paper machine, Thefiller content in the stock was 40% of the dry solids content of thestock. The filler used was ground calcium carbonate. The pH of the stockwas 7.5 and its consistency was 8.3 g/l. Tap water was used as thedilution water. The bentonite used was Hydrocol OA of Allied Colloidsand Altonit SF of Kemira Chemicals. The synthetic metal silicate, inwhich the prevalent cation was magnesium, was Laponite RD (MSRD) ofLaporte. The polymers were Hydrocol 847 of Allied Colloids and PAM1. Theretention results are shown in Table 2. The results are the means of twoparallel tests. The microparticle dosage was 2000 g/tonne. TABLE 2 Totalretention and filler retention results when bentonites of two differentmanufacturers were used, compared with a synthetic metal silicatePolymer dosage, Microparticle Filler Total Polymer g/tonne BentoniteSilicate retention, % retention, % Hydro- 200 Hydrocol OA 34.8 72.0 col847 Hydro- 400 Hydrocol OA 66.5 85.6 col 847 PAM1 200 Altonit SF 31.669.2 PAM1 400 Altonit SF 69.9 87.2 PAM1 200 MSRD 18.5 64.7 PAM1 400 MSRD47.7 77.5

[0043] The results show that the synthetic metal silicate works clearlyless effectively than either bentonite type.

[0044] From the combined results of this example and Example 1 it can beconcluded that the rating order of the three microparticle compositionspresented is synthetic silicate<bentonite<mixture of bentonite andsynthetic metal silicate.

[0045] A mixture of a metal silicate and bentonite thus yields a betterresult than either pure component of the mixture separately.

EXAMPLE 3

[0046] Retention tests were performed mainly in the manner described inExample 1. The stock used was a stock taken from a fine-paper machine,passing to the headbox. The stock sample had been taken just beforeretention aid additions. The filler content in the stock was 38% of thedry solids content of the stock. The filler was precipitated calciumcarbonate. The pH of the stock was 8.2 and its consistency was 7.8 g/l.The bentonite used was Altonit SF of Kemira Chemicals. The syntheticmetal silicate was either Laponite RD (MSRD) of Laporte or itspolyphosphate-modified version Laponite RDS (MSRDS). The polymer wasPAM1, the dosage of which was 400 g/tonne. The proportion of thesynthetic metal silicate in the mixture was 10% and that of bentonitewas 90%. The retention results are shown in Table TABLE 3 Effect of theselection of the synthetic metal silicate on the retention improvementproduced by the mixture Microparticle dosage, Filler retention, Totalretention, Microparticle g/tonne % % Bentonite 1000 65.4 83.0MSRD/bentonite 1000 69.9 86.4 MSDRS/bentonite 1000 69.6 87.3 Bentonite2000 68.9 85.5 MSRD/bentonite 2000 72.0 87.5 MSRDS/bentonite 2000 70.185.8

[0047] Regardless of the synthetic metal silicate used, filler retentionis always better when a mixture is used than when bentonite alone isused as the microparticulate material. The difference caused inretention by different synthetic silicates is slight.

[0048] On the basis of the results shown in this example it was observedthat the type of the synthetic metal silicate used in the mixture washardly significant in terms of retention.

EXAMPLE 4

[0049] The test arrangements were as in Example 3. The proportion ofsynthetic metal silicate in the mixture was 5-10% and the proportion ofbentonite was 90-95%. The results are shown in Table 4. TABLE 4 Effectof the mixing ratio on retention when a mixture of bentonite and MSRD orof bentonite and MSRDS metal silicate is used. Microparticle dosage,Filler Total Microparticle g/tonne retention, % retention, % Bentonite2000 68.9 85.5 MSRD/bentonite 5/95 2000 73.6 86.0 MSRD/bentonite 10/902000 72.0 87.5 MSRDS/bentonite 5/95 2000 71.6 87.3 MSRDS/bentonite 10/902000 70.1 85.8

[0050] According to this example, the mixing ratio hardly affectsretention, and the type of the synthetic metal silicate also does nothave substantial significance.

EXAMPLE 5

[0051] Retention tests were performed using a Moving Belt DrainageTester simulator. The simulator models the forming of a paper, web inconditions resembling web forming in a paper machine so that, during theforming of the web, pulsating scraping of the web and a very high vacuumlevel, typically in the order of −30 kPa, are used. The simulator isdescribed in greater detail in Björn Krogerus's article “Laboratorytesting of retention and drainage”, p. 87 in Leo Neimo (ed.),Papermaking Science and Technology, Part 4, Paper Chemistry, Fapet Oy,Jyväskylä 1999.

[0052] The stock used was, in accordance with Example 1, stock takenfrom a fine-paper machine, passing to the headbox. The stock sample hadbeen taken just before retention aid additions. The targeted vacuumlevel while air was being caused to flow through the sheet was −30 kPa.The effective suction time was 250 ms. The temperature of the stockduring the tests was 45° C. The targeted gramrmage was 80 g/M². Themixing velocities were selected so as to be suitable for the simulator,according to the same principle as that shown in Table 1. The bentoniteused was Altonit SF of Kemira Chemicals. The polymer was PAM1, with adosage of 400 g/tonne. The retention results are shown in Table 5. Theresults are the means of 10 parallel tests. TABLE 5 Retention resultswith different test arrangements when a microparticle mixture accordingto the invention was used Microparticle Total retention, Microparticledosage, g/tonne % Bentonite 2000 90.4 MSRDS/bentonite 5/95 1500 95.0MSRDS/bentonite 5/95 2000 98.5

[0053] The mixtures of a synthetic metal silicate and bentonite usedyielded a clearly better retention result regardless of the dosage thandid bentonite alone.

[0054] A comparison of the results obtained using a test arrangementaccording to Example 1 with the results obtained in the present exampleshows that mixtures of a synthetic metal silicate and bentonite improveretention results as compared with bentonite also when different testarrangements are used.

EXAMPLE 6

[0055] Retention tests were performed mainly in accordance withExample 1. The stock used was an artificial stock prepared in thelaboratory, in which there was used a stock which had been taken from afine-paper machine, passing to the headbox, and which containedprecipitated calcium carbonate as a filler. Thick bleached chemical pineand birch pulps, taken from the same machine, and ground calciumcarbonate were added to the stock. The sample of stock passing to theheadbox had been taken just before retention aid additions. The fillercontent in the stock prepared for the test arrangements was 32% of thedry solids content of the stock. The filler was a mixture ofprecipitated and ground calcium carbonate. The pH of the stock was 8.1and its consistency was 8.1 g/l. Ion-exchanged water was used as thedilution water. The bentonite used was Altonit SF of Kemira Chemicals.The retention results are shown in Table 6a. TABLE 6a Effect of thestock material on retention when artificial stock was used Filler TotalPAM1, Microparticle reten- reten- g/tonne Microparticle dosage, g/tonnetion, % tion, % 400 Bentonite 1000 60.4 84.4 400 MSRD/bentonite 5/951000 63.0 86.8 500 Bentonite 2000 74.5 90.2 500 MSRD/bentonite 5/95 200076.4 94.8

[0056] In addition, retention tests were performed using alaboratory-made artificial stock to which bleached chemical pine andbirch pulps at a ratio of 1:2, taken as a thick pulp from the fine-papermachine, were added. The filler content in the stock was 36% of the drysolids content of the stock. The filler was ground calcium carbonate.The pH of the stock was 7.5 and its consistency was 7.7 g/l.Ion-exchanged water was used as the dilution water. The bentonite usedwas again Altonit SF of Kemira Chemicals and the synthetic metalsilicate was either Laponite RD (MSRD) or RDS (MSRDS) of Laporte. Theproportion of bentonite in the particle mixture was within the range of90-99% and the proportion of metal silicates within the range of 1-10%.The polymer was PAM1, its dosage being 400 g/tonne. The obtained resultsare shown in Table 6b. The results are the means of two parallel tests.TABLE 6b Effect of stock material on retention when an artificial stockcontaining bleached chemical pulp is used Microparticle Filler TotalMicroparticle dosage, g/tonne retention, % retention, % Bentonite 100069.3 89.9 RD 1000 67.5 87.9 RDS 1000 64.5 86.3 MSRD/bentonite 2/98 100071.6 90.9 MSRD/bentonite 5/95 1000 75.3 91.7 MSRDS/bentonite 1/99 100072.8 90.5 MSRDS/bentonite 5/95 1000 73.2 91.1 MSRDS/bentonite 10/90 100074.8 91.6 Bentonite 2000 73.3 91.3 MSRD/bentonite 2/98 2000 77.8 92.6MSRD/bentonite 5/95 2000 77.6 92.9 MSRDS/bentonite 1/99 2000 77.0 91.7MSRDS/bentonite 5/95 2000 78.6 92.6 MSRDS/bentonite 10/90 2000 78.3 92.4

[0057] The results clearly show the superiority of a mixture ofbentonite and a synthetic metal silicate over pure bentonite or puremetal silicates, regardless of the stock material. In addition, theresults show that the superiority of a mixture used according to Example3 was independent of the synthetic metal silicate material in the caseof the stock material concerned. The effect of a varied mixing ratio onthe retention-improving property of the mixture used is slight, as canalso be observed in Example 4, with a slightly different stock material.

EXAMPLE 7

[0058] Retention tests were performed using a Moving Belt DrainageTester simulator, mainly as in Example 6. The stock used was stock takenfrom a machine producing LWC base paper, passing to the headbox. Thestock sample had been taken just before retention aid additions. The pHof the stock was 7.6 and its consistency was 7.5 g/l. The targetedvacuum level while air was being caused to flow through the sheet was−30 kPa. The effective suction time was 250 ms. The temperature of thestock during the tests was 50° C. The targeted grammage was 50 g/m². Themixing velocities were selected so as to be suitable for the simulator,according to the same principle as shown in Table 1. The bentonite usedwas Altonit SF of Kemira Chemicals. The polymer was PAM1, as well asanother cationic polyacrylamide, having a charge of approx. 2 meq/g anda molecular weight of approx. 5 Mg/mol (PAM2). The polymer dosage was300 g/tonne. The filler content in the completed paper sheets wasapprox. 15%. The retention results are shown in Table 7. The results arethe means of ten parallel tests. TABLE 7 Effect of the pulp used onretention improved using a microparticle mixture according to theinvention Microparticle Total Polymer Microparticle dosage, g/tonneretention, % PAM1 Bentonite 1000 66.1 PAM1 MSRD/bentonite 5/95 1000 71.6PAM1 MSRD/bentonite 10/90 1000 70.7 PAM2 Bentonite 1000 68.4 PAM2MSRD/bentonite 5/95 1000 71.0 PAM2 MSRD/bentonite 10/90 1000 70.1

[0059] The obtained results indicate that mixtures of bentonite and asynthetic metal silicate work clearly better than does bentonite alsowith other than fine-paper pulps, in this case a stock containing coarsemechanical pulp.

EXAMPLE 8 (Promoting Effect of MSRD on Action of Hectorite)

[0060] A mixture of MSRD and hectorite has not been compared withhectorite alone within one and the same test series, but the action ofeach has been compared in different series with the action of bentonite,and thus the promoting effect of MSRD on the action of hectorite can beconcluded indirectly by comparing the action of each with the action ofbentonite.

[0061] Retention tests were performed mainly in the manner described inExample 1. However, higher mixing velocities were used in the tests thanin the test of Example 1, since it was desired to examine the action ofmicroparticles at higher shear velocities in order to be closer to theretention values normally appearing in the paper machine. The dosagesequences used are described in Tables 8a and 8b. TABLE 8a Tests withhectorite as the microparticulate material. Stock consistency 8.1 g/lPoint of time, s Event 0 Mixing velocity 1500 rpm. Stock sample (500 ml)into vessel 10 Dosing of polymer 30 Mixing velocity 1980 rpm, 35 Dosingof microparticulate material 45 Collection of filtrate sample

[0062] TABLE 8b Tests using a mixture of MSRD and hectorite as themicroparticulate material. Stock consistency 8.5 g/l Point of time, sEvent 0 Mixing velocity 1500 rpm. Stock sample (500 ml) into vessel 10Dosing of polymer 35 Dosing of microparticulate material 45 Collectionof filtrate sample

[0063] The stock used was a laboratory-made artificial stock, for whichbleached chemical pine and birch pulps (used at a ratio of 1:2) weretaken as a high-consistency pulp from a fine-paper machine (a machinedifferent from that in Example 1). The filler content in the stock was40% of the dry solids content of the stock. The filler was groundcalcuium carbonate. The pH of the stock was 7.5. The consistency intests investigating the action of hectorite in comparison to bentonitewas 8.1 g/l and in tests investigating the action of a mixture of MSRDand hectorite in comparison to bentonite was 8.5 g/l. The dilution waterused was backwater taken from the paper machine and tap water together.

[0064] The hectorite used was Acti-Min 6000H, supplier ITC, Inc. Thebentonite was Altonit SF and the polymer was PAM 1.

[0065] the retention results are shown in Tables 8c and 8d. TABLE 8cAction of hectorite compared with the action of bentonite. The resultsare the means of two parallel tests Microparticle PAM1 dosage, dosage,Filler g/tonne Microparticle g/tonne retention, % 400 Hectorite 100020.4 400 Hectorite 2000 26.5 400 Bentonite 1000 21.6 400 Bentonite 200024.7

[0066] TABLE 8d Action of a mixture of MSRD and hectorite compared withthe action of bentonite. The results are the means of two parellel testsMicroparticle PAM1 dosage, dosage, Filler g/tonne Microparticle g/tonneretention, % 400 MSRD/hectorite 5/95 1000 21.2 400 MSRD/hectorite 5/952000 23.8 400 MSRD/hectorite 10/90 1000 21.4 400 Bentonite 1000 18.9 400Bentonite 2000 20.4

[0067] The filler retention attained with hectorite with a dosage of1000 g/tonne is 94% of the filler retention attained with bentonite whenbentonite is dosed in an equal amount.

[0068] The filler retention attained with hectorite with a dosage of2000 g/tonne is respectively 107% of the filler retention attained withbentonite when bentonite is dosed in an equal amount.

[0069] The filler retention attained with a 5/95 mixture of MSRD andhectorite with a dosage of 1000 g/tonne is 112% of the filler retentionattained with bentonite with the same dosage. The filler retentionattained with a 10/90 mixture of MSRD and hectorite with-a dosage of1000 g/tonne is 113% of the filler retention attained with bentonitewhen bentonite is dosed in an equal amount.

[0070] The filler retention attained with a 5/95 mixture of MSRD andhectorite with a dosage of 2000 g/tonne is 117% of the filler retentionattained with bentonite when bentonite is dosed in an equal amount.

[0071] Thus, at a dosage of 1000 g/tonne the action of hectorite isweaker than that of bentonite, but the actions of mixtures of MSRD andhectorite are clearly better than that of bentonite. At a dosage of 2000g/tonne the action of hectorite is better than that of bentonite, butthat of a mixture of MSRD and hectorite is clearly even better. It canthus be concluded from the results that MSRD helps in improving theaction of also hectorite in retention tests.

1. A method for the production of paper or board in such a manner thatretention aids are added to the stream of stock passing to the papermachine headbox, the stream of stock is directed to the wire, the stockis dewatered in order to form a paper web, and the paper web is dried,characterized in that the retention aids used are a solution of awater-soluble cationic polymer and, in the form of a suspension, amicroparticle mixture containing a swellable clay of the smectite groupand a colloidal synthetic metal silicate, the prevalent cation of thecolloidal synthetic metal silicate being magnesium, wherein themicroparticle mixture contains the swellable clay of the smectite groupin an amount of 85-99% by weight and the colloidal synthetic metalsilicate in an amount of 1-15% by weight,
 2. The method according toclaim 1, characterized in that the said swellable clay of the smectitegroup is for the most part bentonite.
 3. The method according to claim1, characterized in that the said swellable clay of the smectite groupis for the most part hectorite.
 4. The method according to any of thepreceding claims, characterized in that the said retention aids areadded in steps by first adding a solution of the cationic polymer,whereafter there follows a shearing process step to break up flocs, andthereafter the microparticle mixture in suspension form is added.
 5. Themethod according to any of the preceding claims, characterized in thatthe said cationic polymer is a copolymer of acrylamide or methacrylamideand a cationic monomer.
 6. The method according to any of the precedingclaims, characterized in that the molecular weight of the cationicpolymer is at minimum 500,000.
 7. The method according to any of thepreceding claims, characterized in that the said cationic polymer isused in an amount of at minimum 0.02%, preferably 0.03-0.05%, of the drysolids weight of the stock.
 8. The method according to any of thepreceding claims, characterized in that the said microparticle mixtureis used in an amount of at minimum 0.05%, preferably 0.1-0.25%, of thedry solids weight of the stock.
 9. The method according to any of thepreceding claims, characterized in that the suspension-form micropartclemixture is prepared on site from a swellable clay of the smectite groupand a solid colloidal synthetic metal silicate.
 10. The method accordingto any of the preceding claims, characterized in that the stock containscellulose, mechanical pulp or recycled fiber, or various combinations ofthese, as well as filers and additives commonly used in the productionof paper.
 11. The method according to claim 10, characterized in thatthe filler is ground or precipitated calcium carbonate, kaolin, calcinedkaolin, talc, titanium dioxide, gypsum, or a synthetic inorganic ororganic filler.